1. Field of the Invention
This invention relates to a method of removing heavy metals contained in exhaust gas which has been generated by burning a heavy-metal-containing waste liquid discharged in a process of producing organic compounds. More specifically, this invention pertains to a method of separating and removing heavy metals contained in exhaust gas which has been generated by burning a waste liquid containing organic substances and heavy metals discharged accompanied with production of easily-polymerizable substances such as (meth)acrylic acid and/or esters thereof.
2. Description of the Related Art
Industrial waste water containing heavy metals adversely affects the environment. In view of this, it has been a customary practice to discharge waste water after removing heavy metals from the waste water by way of various processes. Particularly, waste water discharged accompanied with production of easily-polymerizable substances such as (meth)acrylic acid contains organic substances and heavy metals. A technique of rendering such waste water harmless involves complicated processes.
Heretofore, catalysts containing heavy metals have been used to react with raw materials when producing easily-polymerizable substances. Polymerization inhibitors containing heavy metals are used as such inhibitors as to prevent polymerization of easily-polymerizable substances in the aforementioned production process. For instance, in the production process of (meth)acrylic acids, catalysts are used in a catalytic gas phase oxidation process in which raw materials are subjected to oxidation with the catalysts to obtain reaction gas. Polymerization inhibitors are used in a process of absorbing acrylic acids by contacting with obtained reaction gas in water, and in a process of obtaining acrylic acids from an aqueous solution of acrylic acid by distillation, stripping, extraction, crystallization, etc. The catalysts and the polymerization inhibitors used in the aforementioned processes contain heavy metals such as copper, manganese, chromium, cobalt, nickel, tin, zinc, and iron. These heavy metals derived from the catalysts and the polymerization inhibitors are removed as impurities in the process of producing acrylic acids, and are discharged through the waste water and waste oil together with other impurities. Burning these waste water and waste oil, however, merely results in emission of exhaust gas containing the heavy metals in the air. In view of this, there has been requested a new technology that enables emission of exhaust gas free from the heavy metals.
As a technology for removing heavy metals from exhaust gas, for example, an indirect cooling method has been proposed. According to this method, exhaust gas obtained by combustion of heavy-metal-containing waste water is introduced in a cooling device where the exhaust gas is cooled by a cooling medium to thereby remove the heavy metals. In the case of the indirect cooling method, however, the heavy metals may likely to adhere or deposit on the inner surface of a gas flow pipe of the cooling device. A long-time operation of the heavy metal removing may lower heat efficiency of the cooling device. Accordingly, operation of the cooling device has to be suspended periodically to remove the heavy metals deposited in the cooling device. Also, an enormous labor has been required to remove the adhered heavy metals from the device.
As another technique of removing heavy metals from exhaust gas, there has been proposed a direct contact cooling method in which a liquid is, for example, rendered into contact with exhaust gas. In this technique, since the liquid and the gas are rendered in direct contact with each other, heavy metals in the exhaust gas can be swiftly transferred to the liquid. However, since the liquid after the direct contact contains the heavy metals, an additional process of separating and removing the heavy metals from the liquid is required to discharge the liquid outside the system of producing acrylic acids in a harmless manner. A technique of efficiently separating and removing the heavy metals from the liquid has not been found.
In view of the above, an object of this invention is to provide a method of removing heavy metals contained in exhaust gas which has been generated by burning a waste liquid containing the heavy metals discharged during a process of producing organic compounds. This method comprises steps of rendering the exhaust gas in contact with a liquid containing an alkali compound to precipitate the heavy metals in the liquid; and adding a coagulant to the liquid to coagulate the heavy metals in the liquid for removal of the heavy metals.
These and other objects, features and advantages of the present invention will become more apparent upon a reading of the following detailed description and accompanying drawing.